• Peter Lund's avatar
    avoid negative (and full-width) shifts in radix-tree.c · 430d275a
    Peter Lund authored
    Negative shifts are not allowed in C (the result is undefined).  Same thing
    with full-width shifts.
    
    It works on most platforms but not on the VAX with gcc 4.0.1 (it results in an
    "operand reserved" fault).
    
    Shifting by more than the width of the value on the left is also not
    allowed.  I think the extra '>> 1' tacked on at the end in the original
    code was an attempt to work around that.  Getting rid of that is an extra
    feature of this patch.
    
    Here's the chapter and verse, taken from the final draft of the C99
    standard ("6.5.7 Bitwise shift operators", paragraph 3):
    
      "The integer promotions are performed on each of the operands. The
      type of the result is that of the promoted left operand. If the
      value of the right operand is negative or is greater than or equal
      to the width of the promoted left operand, the behavior is
      undefined."
    
    Thank you to Jan-Benedict Glaw, Christoph Hellwig, Maciej Rozycki, Pekka
    Enberg, Andreas Schwab, and Christoph Lameter for review.  Special thanks
    to Andreas for spotting that my fix only removed half the undefined
    behaviour.
    Signed-off-by: default avatarPeter Lund <firefly@vax64.dk>
    Christoph Lameter <clameter@sgi.com>
    Cc: Christoph Hellwig <hch@infradead.org>
    Cc: "Maciej W. Rozycki" <macro@linux-mips.org>
    Cc: Pekka Enberg <penberg@cs.helsinki.fi>
    Cc: Andreas Schwab <schwab@suse.de>
    Cc: Nick Piggin <nickpiggin@yahoo.com.au>
    Cc: WU Fengguang <wfg@mail.ustc.edu.cn>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    430d275a
radix-tree.c 27.2 KB